Fuel injection control apparatus for a diesel engine

ABSTRACT

A fuel injection control apparatus for a diesel engine ( 1 ) is provided that eliminates a torque step occurring when shifting from a pilot fuel injection mode to a normal injection mode. After finding a total fuel injection quantity QFIN per cycle (block  101 ), a pilot fuel injection quantity QPILOTB is subtracted therefrom to determine a pilot fuel injection quantity QPILOT and a main fuel injection quantity QMAIN (block  103 ). Corrected pilot fuel injection quantity QPILOTF and corrected main fuel injection quantity QMAINF are determined using a fuel temperature correction coefficient KQTHF and a pilot fuel injection thermal efficiency CEHPILOT (block  104 ) and a main fuel injection thermal efficiency CEHMAIN (block  105 ). The thermal efficiencies are established based on the respective injection timings. A change in torque does not occur because the change in the thermal efficiency caused by the injection timing is offset.

BACKGROUND OF THE INVENTION

[0001] 1. Technical Field

[0002] The present invention generally relates to a fuel injectioncontrol apparatus, especially for a diesel engine. More specifically,the present invention relates to an improvement to a fuel injectioncontrol apparatus that can execute the fuel injection of one cycle inresponse to the operating conditions by dividing the fuel injection intoa main fuel injection and a preceding pilot fuel injection of smallquantity.

[0003] 2. Description of Related Art

[0004] In recent years, the fuel injection modes of diesel engines areequipped with a pilot fuel injection mode that executes the fuelinjection of one cycle by dividing the fuel injection into a main fuelinjection and preceding pilot fuel injection of small quantity. JapaneseLaid-Open Patent Publication No. 9-264159 discloses a fuel injectioncontrol apparatus for a diesel engine equipped with a pilot fuelinjection mode. Under low load conditions, low temperature conditions,and other conditions where the ignitability of the fuel is low, thispilot fuel injection mode improves ignitability and contributes toreducing combustion noise by executing the main fuel injection from astate in which the small quantity of fuel of the pilot fuel injectionhas been combusted. Meanwhile, in regions where the load and enginespeed are somewhat high, the pilot fuel injection is generally notexecuted and the normal injection mode is used. In short, this type ofdiesel engine switches between normal injection mode and pilot fuelinjection mode in response to the operating conditions.

[0005] In view of the above, there exists a need for an improved fuelinjection control apparatus. This invention addresses this need in theprior art as well as other needs, which will become apparent to thoseskilled in the art from this disclosure.

SUMMARY OF THE INVENTION

[0006] It has been discovered that during pilot fuel injection mode thetotal fuel injection quantity Q to be fed to the engine is first foundin response to the operating conditions. FIG. 15 shows the change infuel injection quantity and torque when the engine shifts from the pilotfuel injection mode to the normal injection mode. Next, the optimumpilot fuel injection quantity Q1 is determined and the main fuelinjection quantity Q2 is found by subtracting the pilot fuel injectionquantity Q1 from the total fuel injection quantity Q. Meanwhile, duringthe normal injection mode, the total fuel injection quantity Q isinjected once at a prescribed injection timing.

[0007] In short, even during the pilot fuel injection mode, the quantityof fuel fed into the cylinder during one cycle is the same as during thenormal injection mode. However, the fuel fed by the pilot fuel injectionis injected relatively early and, therefore, generally has an inferiorthermal efficiency to that of the fuel supplied during the main fuelinjection. That is, even if the quantity of fuel is the same, the pilotfuel injection contributes little to torque generation. During thenormal injection mode, in which the fuel is injected with a singleinjection, the thermal efficiency is relatively high because the fuel isinjected at the optimum time. Likewise, the thermal efficiency isrelatively high in the main fuel injection because the fuel is injectedat the optimum time. Therefore, there is a drawback in that, as shown inFIG. 15, the generated engine torque increases and a torque step occurswhen the engine shifts from the pilot fuel injection mode to the normalinjection mode, even if the total fuel injection quantity Q is the same.

[0008] It is feasible that this kind of torque step could be avoided bytaking the value of pilot fuel injection quantity Q1 that is provided inadvance by the shape of the map in response to the operating conditionsand correcting it further in response to the load. However, it would bedifficult to reliably avoid a torque step because the load would changedepending on the air-fuel ratio, supercharge pressure, etc., when theengine switched from the pilot fuel injection mode to the normalinjection mode.

[0009] These and other objects, features, aspects and advantages of thepresent invention will become apparent to those skilled in the art fromthe following detailed description, which, taken in conjunction with theannexed drawings, discloses a preferred embodiment of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Referring now to the attached drawings which form a part of thisoriginal disclosure:

[0011]FIG. 1 is a diagrammatic view of a diesel engine fuel injectioncontrol apparatus in accordance with one embodiment of the presentinvention;

[0012]FIG. 2 is a functional block diagram of an injection quantitycontrol system for the diesel engine fuel injection control apparatusillustrated in FIG. 1;

[0013]FIG. 3 is a characteristic graph showing a torque sensitivitycurve with respect to the injection timing assuming the same injectionquantity;

[0014]FIG. 4 is a characteristic graph showing the relationship betweenthe injection quantity and torque for different injection timings;

[0015]FIG. 5 is a characteristic graph showing the relationship of thethermal efficiency constant with respect to the injection timing;

[0016]FIG. 6 is characteristic graph showing the change in the injectionquantity and the torque when the engine shifts from the pilot fuelinjection mode to the normal injection mode;

[0017]FIG. 7 is a characteristic graph showing the injection pattern andthe center of gravity position or barycenter during the pilot fuelinjection mode;

[0018]FIG. 8 is a characteristic graph showing the relationship betweenthe rail pressure PR and the vertical component of the center of gravityposition or barycenter;

[0019]FIG. 9 is a block diagram showing a correction of the thermalefficiency based on the center of gravity position or barycenter;

[0020]FIG. 10 is a characteristic graph showing the trend of thehorizontal component of the map for the correction of the thermalefficiency based on the center of gravity position or barycenter;

[0021]FIG. 11 is a characteristic graph showing the trend of thevertical component of the map for the correction of the thermalefficiency based on the center of gravity position or barycenter;

[0022]FIG. 12 is a characteristic graph showing the relationship of thethermal efficiency with respect to the injection timing and thesupercharge pressure;

[0023]FIG. 13 is a characteristic graph showing the relationship of thethermal efficiency with respect to the injection timing and the EGRrate;

[0024]FIG. 14 is a characteristic graph showing the relationship of thethermal efficiency with respect to the injection timing and the swirlratio; and

[0025]FIG. 15 is a characteristic graph showing the change in theinjection quantity and the torque when the engine shifts from aconventional pilot fuel injection mode to the normal injection mode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] Selected embodiments of the present invention will now beexplained with reference to the drawings. It will be apparent to thoseskilled in the art from this disclosure that the following descriptionof the embodiments of the present invention is provided for illustrationonly, and not for the purpose of limiting the invention as defined bythe appended claims and their equivalents.

[0027] Referring initially to FIG. 1, the mechanical features of a fuelinjection control apparatus for a diesel engine 1 is illustrated toexplain a first embodiment of the present invention. As explained below,the diesel engine fuel injection control apparatus of the presentinvention executes the fuel injection of one cycle in response to theoperating conditions of the engine 1 by dividing the fuel injection intoa main fuel injection portion and a preceding pilot fuel injectionportion of smaller quantity than the main fuel injection portion. Thefuel injection quantities are corrected based on the thermal efficiencyof the respective injection portion.

[0028] In short, when the engine 1 shifts between the normal injectionmode and the pilot fuel injection mode any change in the thermalefficiency (the degree of contribution to engine torque) from the modechange is offset by the correction of the pilot fuel injection quantityand the correction of the main fuel injection quantity. As a result, thetorque is smoothly continuous.

[0029] The decline in the thermal efficiency of the pilot fuel injectionis caused mainly by the injection timing. The thermal efficiencydeclines when the injection timing deviates from the optimum timing forengine torque generation. Therefore, a fuel injection quantity thattakes this thermal efficiency into account is obtained by adding acorrection based on this injection timing. In one embodiment of thepresent invention, the thermal efficiency is established based on therespective fuel injection timings of the main fuel injection and thepilot fuel injection.

[0030] Moreover, in the present invention, the thermal efficiency can befurther corrected in response to the fuel pressure. If the fuel pressurechanges, the fuel injection rate changes and the thermal efficiency isaffected. Similarly, in the present invention, the thermal efficiency isfurther corrected in response to the supercharge pressure. Also, in thepresent invention, the thermal efficiency can further be corrected inresponse to the exhaust gas recirculation rate. Since the superchargepressure and the exhaust gas recirculation rate also affect the thermalefficiency, and it is preferred to add corrections in response to them.

[0031] Referring to FIG. 1, the diesel engine 1 is equipped with acommon rail type fuel injector device that is equipped with a highpressure supply pump 2, a common rail in the form of an accumulator 3,and an injection nozzle 4 for each cylinder. The injection nozzle 4turns the fuel injection ON and OFF by means of an internal solenoidvalve. A crank angle sensor 6 is provided for detecting the crank angle(CA) and the rotational speed of the engine 1.

[0032] A pressure sensor 7 detects the fuel pressure inside theaccumulator 3. The fuel pressure inside the accumulator 3 is controlledby means of a pressure regulator 8 to a target value corresponding tooperating conditions determined by an engine control unit 20. Asexplained below, the thermal efficiency can be further corrected inresponse to the fuel pressure inside the accumulator 3. If the fuelpressure changes, the fuel injection rate changes and the thermalefficiency is affected.

[0033] The control unit 20 receives input signals from the crank anglesensor 6, a throttle opening sensor 21, a water temperature sensor 22,and a fuel temperature sensor 23. The throttle opening sensor 21 outputsa signal in response to the throttle opening. The water temperaturesensor 22 detects the engine coolant temperature. The fuel temperaturesensor 23 detects the fuel temperature. The control unit 20 uses thesesignals to control the fuel injection quantity and injection timing, aswill be discussed later. The engine 1 is also equipped with asupercharger 13 comprising a turbocharger and a supercharge pressuresensor 5 that detects the supercharge pressure. An EGR valve 12 isprovided in EGR passage 11, which links intake passage 9 with exhaustpassage 10. The EGR valve 12 controls the exhaust gas recirculationamount.

[0034] The control unit 20 preferably includes a microcomputer with aninjection control program that controls the fuel injection quantity andinjection timing as discussed below. The control unit 20 can alsoinclude other conventional components such as an input interfacecircuit, an output interface circuit, and storage devices such as a ROM(Read Only Memory) device and a RAM (Random Access Memory) device. Thememory circuits store processing results and control programs such asones for controlling the fuel injection quantity and injection timingthat are run by the processor circuit. The control unit 20 isoperatively coupled to the supercharge pressure sensor 5, the crankangle sensor 6, the throttle opening sensor 21, the water temperaturesensor 22, and the fuel temperature sensor 23 in a conventional manner.The control unit 20 is capable of selectively controlling the injectionnozzle 4 by turning the fuel injection ON and OFF by means of theinternal solenoid valve in accordance with the injection controlprogram.

[0035] It will be apparent to those skilled in the art from thisdisclosure that the precise structure and algorithms for the controlunit 20 can be any combination of hardware and software that will carryout the functions of the present invention. In other words, “means plusfunction” clauses as utilized in the specification and claims shouldinclude any structure or hardware and/or algorithm or software that canbe utilized to carry out the function of the “means plus function”clause.

[0036] The supercharge pressure sensor 5, the crank angle sensor 6, thethrottle opening sensor 21, the water temperature sensor 22, and thefuel temperature sensor 23 are conventional components that are wellknown in the art. Since the supercharge pressure sensor 5, the crankangle sensor 6, the throttle opening sensor 21, the water temperaturesensor 22, and the fuel temperature sensor 23 are well known in the art,these structures will not be discussed or illustrated in detail herein.Moreover, “sensing means” as utilized in the claims should include anystructure that can be utilized to carry out the function of thesesensors 5, 6, 21, 22, and 23 of the present invention.

[0037] With a diesel engine fuel injection control apparatus inaccordance with the present invention, when the pilot fuel injection isexecuted under certain operating conditions, the pilot fuel injectionquantity and the main fuel injection quantity are corrected to take intoaccount the thermal efficiency, i.e., the degree of contribution toengine torque, of the respective injection. Consequently, when theengine switches between the pilot fuel injection mode and the normalinjection mode, the operability is improved without the occurrence of atorque step. Additionally, worsening of the exhaust performance causedby poor agreement with the exhaust recirculation rate, the superchargepressure, etc., which are established in reference to a prescribedtorque generation, can be avoided.

[0038] In particular, by correcting each injection quantity based on thefuel injection timing, the optimum correction can always be executed inaccordance with the pilot fuel injection timing and main fuel injectiontiming, which are established variably in response to the operatingconditions.

[0039] Also, by further correcting the thermal efficiency in response toa variety of parameters, the pilot fuel injection quantity and the mainfuel injection quantity can be brought to even better suited values.

[0040] Referring now to FIG. 2, a block diagram illustrates theinjection quantity control system 100 that forms an injection quantitycontrol program executed by the control unit 20. The details of theinjection quantity control program of the present invention areexplained below in reference to this block diagram.

[0041] First, as shown in block 101 of FIG. 2, the basic total fuelinjection quantity QFIN per cycle is calculated in accordance with thecurrent operating conditions at the time. The basic total fuel injectionquantity QFIN is generally determined based on the engine speed NEdetected by the crank angle sensor 6 and the throttle opening detectedby the throttle opening sensor 21. In other words, the basic total fuelinjection quantity QFIN is the total fuel quantity required per cyclefor the current operating conditions, e.g. current injection timing andcurrent engine speed. The crank angle sensor 6 and the throttle openingsensor 21 form total fuel injection quantity determination means fordetermining the total fuel injection quantity QFIN per cycle. However,the “total fuel injection quantity determination means” as utilized inthe claims should include any structure that can be utilized to carryout the function of these sensors 6 and 21 of the present invention.

[0042] Next, if the injection quantity control system 100 is in thepilot fuel injection mode, the required pilot fuel injection quantityQPILOTB is found using a prescribed map based on the aforementioned fuelinjection amount QFIN, the engine speed NE, and the water temperatureTHW, as indicated in block 102. The prescribed map for the requiredpilot fuel injection quantity QPILOTB is stored in the control unit 20.If the injection quantity control system 100 is in normal injection mode(during which pilot fuel injection is not conducted), then the value ofthe pilot fuel injection quantity QPILOTB will become Whether to use thepilot fuel injection mode or the normal injection mode is determinedbased on the current detected operating conditions. More specifically,use of the pilot fuel injection mode or the normal injection mode isbased on a prescribed mode map stored in the control unit 20. Thisprescribed mode map uses the aforementioned fuel injection quantity QFINand the engine speed NE as detected parameters to determine the mode ofoperation. In general, the pilot fuel injection mode is only used inregions of low speed and low load. As previously mentioned, if thenormal injection mode is determined based on the current detectedoperating conditions, then the value of the pilot fuel injectionquantity QPILOTB will become “0”.

[0043] After finding the pilot fuel injection quantity QPILOTB in block102 as just described, the initial distribution of the fuel injectionquantities is determined in block 103. In other words, theaforementioned fuel injection quantity QFIN is split into two portions,namely the main fuel injection quantity QMAIN and the pilot fuelinjection quantity QPILOTB. The value of the main fuel injectionquantity QMAIN is obtained by subtracting the pilot fuel injectionquantity QPILOTB (block 102) from the fuel injection quantity QFIN(block 101), while the pilot fuel injection quantity QPILOTB is used asthe pilot fuel injection quantity QPILOT.

[0044] Next, as shown in block 104, the pilot fuel injection quantityQPILOT is corrected to determine the corrected or final pilot fuelinjection quantity QPILOTF. Here, corrections are determined foradjusting the pilot fuel injection quantity based on the fueltemperature THF and based on the thermal efficiency for the particularengine during pilot fuel injection and. More specifically, as shown inblock 106, a prescribed fuel temperature map stored in the control unit20 is used to determine the fuel temperature correction coefficientKQTHF corresponding to the fuel temperature THF detected by the fueltemperature sensor 23. As shown in block 107, a prescribed thermalefficiency map stored in the control unit 20 is used to find the pilotfuel injection thermal efficiency CEHPILOT corresponding to the pilotfuel injection timing ITPILOT, which based on the crank angle detectedby the crank sensor 6. Then, the final pilot fuel injection quantityQPILOTF is found using the following equation.

QPILOTF=KQTHF×QPILOT×(1/CEHPILOT).  Equation 1:

[0045] Similarly, as shown in block 105, corrections are determinedbased on the fuel temperature THF and based on the thermal efficiencyfor the particular engine during main fuel injection. These correctionsare executed against the aforementioned main fuel injection quantityQMAIN to obtain the corrected or final main fuel injection quantityQMAINF. The fuel temperature correction coefficient KQTHF is used inblock 106 in the same manner as described above. Then, as shown in block108, a prescribed thermal efficiency map stored in the control unit 20is used to find the main fuel injection thermal efficiency CEHMAINcorresponding to the main fuel injection timing ITMAIN, which is basedon the crank angle detected by the crank sensor 6. The final main fuelinjection quantity QMAINF is then found using the following equation.

QMAINF=KQTHF×QMAIN×(1/CEHMAIN).  Equation 2:

[0046] Here, the pilot fuel injection timing ITPILOT and the main fuelinjection timing ITMAIN are each determined based on a prescribedinjection timing maps stored in the control unit 20. The prescribedinjection timing maps have the fuel injection quantity QFIN and theengine speed NE as detected parameters to determine the pilot fuelinjection timing ITPILOT and the main fuel injection timing ITMAIN.During the normal injection mode, the main fuel injection timing ITMAINis equivalent to the injection timing IT. However, the pilot fuelinjection mode and the normal injection mode each use differentinjection timing maps and control the optimum values for the respectiveconditions.

[0047] The pilot fuel injection thermal efficiency CEHPILOT and the mainfuel injection thermal efficiency CEHMAIN are explained based on FIGS. 3to 5. As explained earlier, even when the same fuel is injected, thecontribution to torque generation differs depending on the injectiontiming. FIG. 3 shows the injection timing torque sensitivity curveassuming the same injection quantity. The generated torque is maximum inthe vicinity of top dead center (TDC) and decreases for both earlier andlater timings. Therefore, as shown in FIG. 5, the thermal efficiencyconstant CEH is defined to be “1” at the point where the torquegeneration is maximum. The thermal efficiency maps of the previouslymentioned blocks 107 and 108 are established according to thecharacteristic shown in FIG. 5. Thermal efficiencies CEHPILOT andCEHMAIN are each provided as a value less than or equal to “11” inaccordance with the injection timing, which is based on the crank angledetected by the crank sensor 6. FIG. 4 shows an example of therelationship between the injection quantity and the generated torque forinjection timings at 10° BTDC and 50° BTDC.

[0048] As explained previously, the final pilot fuel injection quantityQPILOTF and the final main fuel injection quantity QMAINF are correctedaccording to the difference in the thermal efficiency that is based onthe difference in the injection timing. As a result, for example, theinjection quantities change as shown in FIG. 6 when the engine 1 shiftsfrom the pilot fuel injection mode to the normal injection mode inresponse to light acceleration. The sum of the final pilot fuelinjection quantity QPILOTF and the final main fuel injection quantityQMAINF is larger than the basic injection quantity QFIN so that thedecline in the thermal efficiency is offset. Therefore, a step-liketorque increase does not occur when the engine 1 shifts from the pilotfuel injection mode to normal injection mode. Furthermore, in thisembodiment, the basic injection quantity QFIN (equivalent to main fuelinjection quantity QMAINF) is corrected based on the thermal efficiencyCEHMAIN (block 105 in FIG. 2) even during normal injection mode.Consequently, the final injection quantity is slightly larger than thebasic injection quantity QFIN.

[0049] In diesel engines the “injection timing” is generally defined bythe injection start timing, but even if the injection start timing isthe same, the average thermal efficiency will differ if there aredifferences in the length of the injection period. Regardingrepresenting the thermal efficiency, the crank angle position at themiddle of the injection period is preferred over the injection starttiming.

[0050] Next, an even more precise method is described for determiningthe values for the pilot fuel injection thermal efficiency CEHPILOT andthe main fuel injection thermal efficiency CEHMAIN that are used in theinjection quantity control system 100 to determine the final pilot fuelinjection quantity QPILOTF and the main fuel injection quantity QMAINFof FIG. 2. In particular, the thermal efficiency is further corrected inresponse to the fuel pressure. If the fuel pressure changes, the fuelinjection rate changes and the thermal efficiency is affected. The fuelpressure inside the accumulator of an accumulator (common rail) typefuel injector device is generally variably controlled. Consequently, itis preferable to add a correction in response to the fuel pressure.

[0051]FIG. 7 shows the fuel injection pattern during the pilot fuelinjection mode. In FIG. 7, the horizontal axis is the crank angle θ,while the vertical axis is the injection rate dQ/dθ. In this kind ofinjection pattern diagram, the areas of the regions indicated as thepilot fuel injection and the main fuel injection correspond to eachrespective injection quantity. Therefore, when the center of gravityposition or barycenter G of each region is estimated, the center ofgravity position or barycenter G represents the thermal efficiency ofthe respective injection. Thus, it is more preferable for the thermalefficiency to be determined based on this center of gravity position orbarycenter G instead of based simply on the injection timing.

[0052] The injection periods LP and LM of each injection are roughlyproportional to the period during which the solenoid valve of theinjection nozzle 4 is energized for each mode. Thus, the crank angleposition of the center of gravity position or barycenter G can be foundby adding one half of the energized period to the injection start time.Meanwhile, the vertical position of the center of gravity position orbarycenter G in FIG. 7 can be calculated as a function of the railpressure PR, as shown in FIG. 8. Specifically, the vertical position ofthe center of gravity position or barycenter G can be calculated as afunction of the rail pressure PR, because the injection rate dQ/dθ inthe injection pattern is roughly proportional to the fuel pressure (railpressure PR) of the accumulator 3.

[0053] Thus, as shown in FIG. 9, the value of the thermal efficiencyCEHF can be brought to an even higher degree of precision by finding thecorrection coefficient KC1 from a prescribed thermal efficiency mapbased on the center of gravity position or barycenter G. In particular,the value of the thermal efficiency CEHF can be obtained by multiplyingthe previous thermal efficiencies CEH for the pilot fuel injectionthermal efficiency CEHPILOT and the main fuel injection thermalefficiency CEHMAIN by the same correction coefficient KC1. Regarding thetrends of the thermal efficiency map in FIG. 9, the trend of thehorizontal component (crank angle) is shown in FIG. 10, while the trendof the vertical component (injection rate dQ/dθ) is shown in FIG. 11.Thus, by using these thermal efficiency map of FIG. 9, more precisethermal efficiencies CEHPILOT and CEHMAIN are obtained for use in theinjection quantity control system 100 to determine the final pilot fuelinjection quantity QPILOTF and the main fuel injection quantity QMAINFof FIG. 2 as discussed above.

[0054] The thermal efficiency is also affected by the superchargepressure, theEGR rate, the swirl ratio, etc. Therefore, the precisioncan be improved even further by adding corrections with respect to theseitems. FIG. 12 shows an example of a thermal efficiency map that usesthe injection timing IT and the supercharge pressure as detectedparameters. FIG. 13 shows an example of a thermal efficiency map thatuses the injection timing IT and the EGR rate as detected parameters.FIG. 14 shows an example of a thermal efficiency map that uses theinjection timing IT and the swirl ratio as detected parameters.Corrections of even higher precision can be achieved by using thesethermal efficiency maps to find the thermal efficiencies CEHPILOT andCEHMAIN for determining the final pilot fuel injection quantity QPILOTFand the main fuel injection quantity QMAINF by using the injectionquantity control system 100 of FIG. 2 as discussed above. Furthermore,these corrections can be combined as appropriate.

[0055] The term “configured” as used herein to describe a component,section or part of a device includes hardware and/or software that isconstructed and/or programmed to carry out the desired function. Theterms of degree such as “substantially”, “about” and “approximately” asused herein mean a reasonable amount of deviation of the modified termsuch that the end result is not significantly changed. For example,these terms can be construed as including a deviation of at least ±5% ofthe modified term if this deviation would not negate the meaning of theword it modifies.

[0056] This application claims priority to Japanese Patent ApplicationNo. 2000-310725. The entire disclosure of Japanese Patent ApplicationNo. 2000-310725 is hereby incorporated herein by reference.

[0057] While only selected embodiments have been chosen to illustratethe present invention, it will be apparent to those skilled in the artfrom this disclosure that various changes and modifications can be madeherein without departing from the scope of the invention as defined inthe appended claims. Furthermore, the foregoing description of theembodiments according to the present invention are provided forillustration only, and not for the purpose of limiting the invention asdefined by the appended claims and their equivalents. Thus, the scope ofthe invention is not limited to the disclosed embodiments.

1. A fuel injection control apparatus for a diesel engine, comprising: asensing section configured to receive a signal indicative of selectedoperating condition of a diesel engine; a fuel injection calculationsection configured to calculate a pilot fuel injection quantity for apilot injection and a main fuel injection quantity for a main injectionthat occurs following the pilot injection, in response to the signalindicative of the selected operating condition; a thermal efficiencycorrection section configured to correct at least one the pilot fuelinjection quantity and the main fuel injection quantity based on arespective thermal efficiency during the pilot injection and the maininjection; and a fuel injector configured to carry out the pilotinjection and the main injection in response to the pilot fuel injectionquantity and the main fuel injection quantity at least one of which arecorrected in the thermal efficiency correction section.
 2. A fuelinjection control apparatus for a diesel engine, comprising: amicroprocessor programmed to: calculate a total fuel injection quantityto be injected in a cycle based on an operation condition of the dieselengine, divide total fuel injection quantity into a pilot fuel injectionquantity for a pilot injection and a main fuel injection quantity for amain injection, and correct the pilot fuel injection quantity and themain fuel injection quantity based on a pilot thermal efficiency of thepilot injection and a main thermal efficiency of the main injectionrespectively; and a fuel injector configured to carry out the pilotinjection and the main injection in response to the corrected pilot fuelinjection quantity and the corrected main fuel injection quantity.
 3. Afuel injection control apparatus as recited in claim 2, wherein thepilot thermal efficiency is established based on a pilot fuel injectiontiming.
 4. A fuel injection control apparatus as recited in claim 2 or3, wherein the main thermal efficiency is established based on a mainfuel injection timing.
 5. A fuel injection control apparatus as recitedin claim 4, wherein at least one of the pilot thermal efficiency and themain thermal efficiency are corrected based on a respective fuelinjection period thereof.
 6. A fuel injection control apparatus asrecited in claim 4 or 5, wherein at least one of the pilot thermalefficiency and the main thermal efficiency is corrected in response to afuel pressure provided to the fuel injector.
 7. A fuel injection controlapparatus as recited in any one of claims 4 to 6, wherein at least oneof the pilot thermal efficiency and the main thermal efficiency iscorrected in response to an intake air pressure introduced in acombustion chamber of the diesel engine.
 8. A fuel injection controlapparatus as recited in any one of claims 4 to 7, wherein at least oneof the pilot thermal efficiency and the main thermal efficiency iscorrected in response to a parameter related to an exhaust gasrecirculation rate of the diesel engine.
 9. A fuel injection controlapparatus as recited in any one of claims 2 to 8, wherein the pilot fuelinjection quantity and the main fuel injection are corrected to begreater as the pilot thermal and the main thermal efficiency becomessmaller respectively.
 10. A fuel injection control apparatus as recitedin any one of claims 2 to 9, wherein the microprocessor is furtherprogrammed to switch between a normal injection mode having the maininjection and a pilot injection mode having both the pilot injection andthe main injection in accordance with the operation condition of thediesel engine.
 11. A fuel injection control apparatus as recited inclaim 10, wherein the pilot injection mode is performed in a low-speedand low-load region, and the normal injection mode is performed otherthan the low-speed and low-load region.
 12. A fuel injection controlapparatus as recited in any one of claims 2 to 11, wherein the pilotthermal efficiency and the main thermal efficiency are set smallerrespectively as the position of the fuel injection thereof in a crankangle becomes advanced relative to a top dead center position of pistonof the diesel engine.
 13. A fuel injection control apparatus as recitedin any one of claims 2 to 12, wherein the pilot thermal efficiency andthe main thermal efficiency are set smaller respectively as the positionof the fuel injection thereof in a crank angle becomes retarded relativeto a top dead center position of piston of the diesel engine.
 14. A fuelinjection control apparatus as recited in claim 2, wherein at least oneof the pilot thermal efficiency and the main thermal efficiency isestablished based on a pilot injection barycenter and a main injectionbarycenter respectively.
 15. A fuel injection control apparatus asrecited in claim 14, wherein the pilot injection barycenter and the maininjection barycenter in a crank angle are calculated respectively byadding one second of injection period in the crank angle to an injectionstart timing in crank angle.
 16. A fuel injection control apparatus asrecited in claims 14, wherein the pilot injection barycenter and themain injection barycenter in an injection rate direction are calculatedas a function of a fuel pressure.
 17. A fuel injection control apparatusfor a diesel engine comprising: calculating means for calculating atotal fuel injection quantity to be injected in a cycle based on anoperation condition of the diesel engine; dividing means for dividingtotal fuel injection quantity into a pilot fuel injection quantity for apilot injection and a main fuel injection quantity for a main injection;correcting means for correcting the pilot fuel injection quantity andthe main fuel injection quantity based on a pilot thermal efficiency ofthe pilot injection and a main thermal efficiency of the main injectionrespectively; and an injector for carrying out the pilot injection andthe main injection in response to the corrected pilot fuel injectionquantity and the main fuel injection quantity.
 18. A fuel injectioncontrol apparatus as recited in claim 17, wherein the pilot thermalefficiency is established based on a pilot fuel injection timing.
 19. Afuel injection control apparatus as recited in claim 17 or 18, whereinthe main thermal efficiency is established based on a main fuelinjection timing.